The field of this invention is guided, spin stabilized ballistic missiles or shells. It has been found that by providing terminal guidance to conventional artillery shells in the 155 mm to 8 inch range, the accuracy can be significantly improved. For example, the effects of imprecise meteorological data, muzzle velocity variations and target movement can be corrected with a target seeking guidance system. Such shells comprise canard surfaces for steering control, an electronic target acquisition and guidance system, and a set of tail fins for lift augmentation. The tail fins however must be retracted for storage and handling and while the shell is traversing the barrel, and must be deployed just after muzzle exit. The present invention provides an artillery shell of the type described with a set of tail fins which are retracted at all times prior to muzzle exit and which are automatically deployed or extended immediately after the round leaves the muzzle, through the combined action of the change in pressure experienced upon muzzle exit and the centrifugal force on the retracted fins due to shell spin. The mechanism provides a simple, reliable and inexpensive means for automatically deploying the fins.